1. Field of the Invention
This invention relates generally to tube and fin heat exchangers, and in particular, to manufacturing processes and equipment for producing tube and fin heat exchangers, such as for HVAC systems.
2. Description of the Prior Art
As illustrated in FIG. 1, a typical tube and fin heat exchanger (10) consists of a stack of generally planar metallic fins (12) sandwiched between a top end plate (14) and a bottom end plate (16). The terms “top” and “bottom” used for designating heat exchanger end plates are derived based on the heat exchanger orientation during expansion in a vertical hairpin expander press, as described below. The “top” and “bottom” designations are not necessarily indicative of the heat exchanger orientation in any particular installation.
The fins (12) have a number of collared holes (18) formed therethrough, and the top and bottom end plates (14, 16) have corresponding holes (20) formed therethrough. When the fins (12) and end plates (14, 16) are stacked, the holes (18, 20) are in axial alignment for receiving a number of U-shaped hairpin tubes (“hairpins”) (22) through the stack. Hairpins (22) are formed by bending lengths of small tubes, typically copper, aluminum, steel or titanium, 180 degrees around a small diameter mandrel. The hairpin tubes (22) are fed, or laced, through the loosely-stacked assembly of fins from the bottom end plate (16) so that the open ends (26) of the hairpin tubes (22) extend beyond the top end plate (14). The top end plate (14) is slipped over the open ends (26) of the hairpins (22), and the hairpins (22) are mechanically expanded from within to create a tight fit with the fins (12). Finally, return bend fittings (24) are soldered or brazed to the open ends (26) of the hairpin tubes (22) to create a serpentine fluid circuit through the stack of fins (12).
FIG. 2 is a flow chart diagram that describes the manufacturing process of prior art used to mass produce tube and fin heat exchangers. Referring to both FIGS. 1 and 2, as shown in step (50), fins (12) are formed by a stamping process in a fin press, such as those produced by Burr Oak Tool, Inc. of Sturgis, Mich. Fin stock is delivered to a press in a roll of sheet metal. Various metals, heat treatments, and thicknesses may be used, but aluminum is the general industry selection. Fin stock is paid out from an uncoiler, lubricated, then fed through a press, where a die draws, details, punches collared holes, and cuts fins to a desired length and width. Stamping generally occurs in several stages. At the back end of the fin press, fins index out of the die under a vacuum hood, where a pressure differential holds them in place until discontinued, at which time (sometimes mechanically assisted by wire) the fins drop from the vacuum hood on to collection rods that pass through collared holes of the fins. The collection rods are mounted on to collection tables. Once having a full stack of fins, the collection table is removed from the back-end of the fin press. Drop rods are inserted into the fin stacks to keep the stacks intact. Operators lift stacks of fins by placing their hand at the bottom of the stack and physically lift the stack up off the collection table rods. Fin stacks then are staged for the lacing process as depicted by element (56) in FIG. 2.
As shown by step (52) in FIG. 2, the heat exchanger top and bottom end plates (14, 16) are manufactured in a stamping process that is independent of the fin stamping process (50). The end plates are typically made of a fairly stiff sheet metal. The end plates (14, 16) may also each include bends that form a channel or similar profile to provide strength and rigidity. Holes (20), which align with the collared holes (18) of the fins (12), are punched through the end plates by a press and die.
The hairpin tubes (22) are manufactured in process step (54). Referring to FIGS. 2 and 3, hairpins are typically formed in a hairpin bender machine (88), using machines such as vertical bend hairpin bender, manufactured by Burr Oak Tool, Inc. of Sturgis, Mich., to form multiple hairpins at a time. Depending on the outer diameter of the stock tubing, commonly up to six lines of tubing are typically processed simultaneously in a single hairpin bender machine (88).
A typical vertical bend hairpin bender machine (88) consists of three sections—the tubing pay-out section (90), a feeder section (92), and a bender section (94). U.S. Pat. No. 6,354,126, issued to Small, et al. and entitled “Tube Straightener and Drive Therefor,” describes a typical feeder section (92), and the patent is incorporated herein in its entirety by reference. U.S. Pat. No. 5,233,853, issued to Jones G. Milliman and entitled “Stretch Straightening Hairpin Bender,” describes a typical bender section, and it is also incorporated herein in its entirety by reference.
The pay-out section (90) includes a coil stand (96), also known as an uncoiler, for supporting multiple tubing spools or bare-pack tubing coils. As stock tubing (100) is paid out from bare-pack coils or spools at the pay-out section (90), the stock tubing (100) will typically contain bends and may be out-of-round at times. Thus, the feeder section usually includes correction rollers for reforming the stock tubing back to nominal dimensions.
A more detailed view of the feeder section is shown in FIG. 4. Cross-axis rollers (102) correct ovality, eccentricity, and out-of-round conditions of the stock tubing (100), returning it to a circular profile. Next, a pair of pre-size rollers (104) typically surrounds and rolls the stock tubing (100) to return any portion of stock tubing that has a slightly larger than normal outer diameter to its nominal size. In other prior art hairpin bender configurations, a stationary pre-sizing die (not illustrated) is used in place of the pre-size roller pair (104). A final pair of offset straightening rollers (106) ensures the stock tubing (100) is straight and true.
After the trio of correction rollers (102, 104, 106), a pair of conveyor belts (108) clamps the stock tubing and drives the tubing through the hairpin bender machine (88). Each line of stock tubing (100) being processed by the hairpin bender (88) is fed by the feed belt assembly (108) over a boom (110), a bend arbor clamp (112) and mandrel tip rods (114) in bender section (96). Tube draw for each tubing line continues until that tubing contacts a switch tower. Once all of the tubes have contacted their respective switch tower, the bend arbor clamp (112) engages and a tube cutter head assembly (98), located at the end of the feeder section (92), cuts and reams the tubes. Mandrel tips are extended, and boom (110) actuates, bending the cut tube sections 180 degrees about a mandrel (115), thus creating the hairpin tubes (22). Once the boom (110) actuates its limit switch (not shown), indicating a complete bend, the bend arbor clamp (112) is released, and a stripper assembly (not illustrated for simplicity) pushes the hairpin tubes (22) out of the boom (110) and off of the mandrel tip rods (114), where the hairpins (22) then fall into catch arms (also not illustrated for simplicity). The hairpins (22) are removed from the catch arms and staged in large racks for the lacing process as depicted by element (56) in FIG. 2.
As shown in step (56) of FIG. 2, the lacing process is that process in which stacks of fins (12), the bottom end plate (16), and the hairpins (22) are assembled together, typically by hand. Fin stacks are laid out on a lacing table, one stack at a time. Drop rods are removed from each fin stack as multiple stacks are assembled together on the table to form a contiguous slab of fins. The heat exchanger bottom end plate (16) is added to one end of the slab, and it is temporarily held in place with rods. These rods also help maintain fin alignment until an adequate number of the hairpin tubes have been laced through the assembly to maintain alignment. Hairpins (22) are typically hand-laced through the bottom end plate (16) into the slab of fins (12) one at a time by an operator who manually finesses them in. For conventional diameter hairpins and fins of the prior art, for example ⅜ inch diameter hairpins, lacing is a simple process, taking on average no more than five seconds per hairpin.
At this stage of assembly, the heat exchanger consists of stacks of fins (12) and a bottom end plate (16) loosely held together by hairpins (22) passing transversely through the assembly. As shown in step (58) of FIG. 2, in order to form tight metal-to-metal interfaces between the tubes and the fins of the heat exchanger so that efficient conductive heat transfer paths are created between the tubes and the fins, the hairpins (22) are expanded within the fins to create an interference fit.
The laced heat exchanger assembly is placed within a hairpin expander machine (150), and the top end plate (14) is slipped over the open ends (26) of the hairpins (22). FIG. 6 shows a typical hairpin expander (150) of prior art into which a heat exchanger assembly is vertically placed with the open ends (26) of the hairpins (22) facing upwards. Referring to FIGS. 1 and 6, hairpin expander (150) has bullets (152) located at the ends of long rods (154) for passing through the open ends (26) of the hairpins (22). Multiple bullets (152) and rods (154), two for each hairpin (22), are typically provided for simultaneously expanding all of the hairpins (22). Each bullet (152) is sized to have an outer diameter larger than the inner diameter of the hairpin tubes (22). The expander has a hydraulic ram (151), that acts upon a pressure plate (153) which in turn drives rods (154). As the expander (150) presses the bullets (152) into the hairpins (22), the bullets (152) expand the hairpins (22) into a tight, interference-fit engagement with the fins (12).
Expansion of the diameter of the hairpins causes axial shrinkage of the hairpins. Typically, about a 3-5 percent reduction in hairpin length occurs during the prior art expansion process used with conventional hairpin tube diameters (e.g. ⅜ inch). In the earlier vintage hairpin expanders (150), the heat exchanger stack is supported in the expander at the bottom end plate (16) and at the bends (23) of the hairpins (22) by a bolster plate (156). The hairpin bends (23) are supported by a receiver plate or cradle plate (158) that has semicircular grooves (161) cut therein to accommodate them. The receiver or cradle plate (158) is in turn supported on the bolster plate (156). The stack of fins (12) and the top end plate (14) “float” or rest on the bottom end plate (16). As the hairpins are expanded, the hairpins (22) are under compressive forces. Therefore, expander (150) includes a fixture (160) mounted to the expander frame, into which the heat exchanger is placed. Fixture (160) includes front and back plates (162) that laterally support the stack of fins (12) to prevent them from buckling during the expansion. Side rails (164) may be included in fixture (160) for making it easier to center the heat exchanger within the expander (150).
Because the top end plate (16) becomes initially fixed in position near the tips (26) of the hairpins (22) after expansion is first commenced, there is a concomitant shrinking and tightening of the stack of fins and end plates due to the longitudinal shrinkage of the hairpins (22) during expansion. Even with attempts to predict and compensate for the shrinkage of the hairpins with this type of expander, the process still results in heat exchangers having large dimensional variances.
The more advanced expanders of prior art employ a coil shrink rate control feature that forces all of the hairpin tubes to shrink at the same rate. With this type of expander, both the top and bottom end plates (14, 16) are held fixed within the fixture (160) at the desired dimensions, thus providing a finished heat exchanger product of having higher dimensional tolerances. The hairpin bends (23) are supported in a cradle or receiver plate (158), which is in turn supported by the bolster plate (156). During initial expansion, the hairpins (22) are in compression. However, because the top end plate is held fast within the fixture (160), after the bullets have passed through the top end plate (14), securing the top end plate near the upper ends (26) of the hairpins (22), the compressive hairpin force becomes a tensile force as the hairpin bends (23) contract and pull away from the cradle or receiver plate (158). The hairpins (22) are held by the top end plate (14) in the fixture (160) during expansion, and as the hairpins (22) contract in length, the hairpin tubing below the bullets (152) slide upwards within the stack of fins (12), moving the hairpin bends (23) toward the bottom end plate (16).
It is possible that the tensile force exerted on the hairpins (22) at the top end plate (14) by the bullets (152) during expansion may exceed the strength of the interference fit that holds the hairpins (22) in the holes (20) of the top end plate (14). If this happens, damage to the heat exchanger will occur. Therefore, with the controlled-shrink-rate expander, the bolster plate (156), which carries the cradle or receiver plate (158), is designed and arranged to move upwards at the same rate as the hairpin bends (23) move upwards, thus continuing to provide support of the hairpins.
U.S. Pat. No. 4,780,955 issued to Stroup on Nov. 1, 1988 and entitled “Apparatus for Making a Tube and Fin Heat Exchanger” describes an expander that employs coil shrink rate control, and the patent is incorporated herein in its entirety by reference. The '955 patent teaches that the bolster plate may be mechanically driven as a function of the position of the ram cylinder that drives the pressure plate, the rods and the expansion bullets. For example, for every inch of downward travel of the ram cylinder, a cam arrangement (not illustrated) drives the bolster upward 0.03 inches. The '955 patent also discloses a second arrangement in which a pneumatic actuator (not illustrated) drives the bolster plate upward. The pneumatic actuator force is manually selected by the operator so as to approximately balance the force applied to the hairpins by the bullets.
In the controlled-shrink-rate expander of prior art described above, the upward movement of the bolster plate and receiver or saddle plate tends to apply an upward force on the heat exchanger bottom end plate. The bottom end plate of the heat exchanger is held fast within the expander fixture by a piano hinge clamp arrangement, as illustrated in FIGS. 7-8. FIG. 7 illustrates the bottom end of a heat exchanger (10) that is to be positioned within the expander for expanding the hairpins into an interference-fit engagement within the stack of fins (12). The hairpin bends (23) are shown extending beyond the bottom end plate (16) of the laced heat exchanger. The heat exchanger fins are not illustrated for simplicity. Fixture base plate (165) receives and positions the heat exchanger bottom end plate (16) during the shrink-rate-controlled expansion process described above. Base plate (165) may have a recessed seat (166) into which bottom end plate (16) is received. A slot (167) is formed through base plate (165) for allowing the hairpin bends (23) to pass therethrough and to be received into the receiver or cradle plate (158). Two piano hinge clamps (168) (only one is shown for simplicity) are attached to fixture base plate (165) and are arranged to be folded over the seated bottom end plate (16), retaining the bottom end plate (16) in the recessed seat (166) of the fixture plate (165). A pair of latch mechanisms (170) are secured to studs (172) in fixture plate (165). The latch mechanisms (170) are rotatable for locking the piano hinge clamps (168) into the folded clamping position or for allowing the piano hinge clamps (168) to swing freely.
FIG. 8 illustrates the piano hinge clamp arrangement of the fixture base plate (165) of FIG. 7, with the heat exchanger bottom end plate (16) seated in the recessed seat (166) of the base plate (165). The piano hinge clamps (168) are folded over the heat exchanger bottom end plate (16), holding the end plate fast to fixture base plate (165). The latches (170) are rotated to keep the piano hinge clamps (168) in the downward, folded position.
Referring back to FIGS. 1 and 2, after the expansion process, a number of short return bend tubes (24) are soldered, brazed or welded to the open ends (26) of the hairpins (22) to create one or more long serpentine circuits from the hairpins for fluid flow. Additionally, one or more cross-over tubes (not shown), which connect various hydraulic circuits, may be soldered, brazed or welded to the open ends (26) of the hairpins (22). In the typical manufacturing process used for manufacturing prior art ⅜ inch tubing heat exchangers, for example, the return bends (26) and the cross-over tubes are brazed to the heat exchanger (10) in an autobrazing process, depicted as step (60), in the flowchart of FIG. 2. The tubes are hand-assembled with brazing rings, and the heat exchanger is run through a furnace, wherein the joints are brazed.
After the autobrazing step (60) in the heat exchanger manufacturing process of prior art, a leak check (62) is performed. For each circuit, one end is plugged while a pressure-decay monitoring device is connected to the other end. If the circuits hold pressure, there are no leaks.
Finally, for heat exchangers used in HVAC systems, in step (64), subcooler, liquid and suction manifolds are manually brazed to the heat exchanger circuits.
There is concern with the effects of R22 refrigerant in depleting the ozone layer, and so the new HVAC systems are designed to use R410 refrigerant. R410 refrigerant systems operate at higher pressures than their R22 counterparts. Higher operating pressure allows the use of smaller diameter tubing in heat exchanger coils of condensers and heat pumps. Smaller diameter tubing provides a better ratio of heat transfer surface areas, has merits in terms of pressure drop on the air side because of reduced form drag, and requires less material to provide the same amount of heat transfer surface area, which is especially attractive from a commercial perspective. Consequently, strong desire exists among HVAC manufacturers to design manufacturing processes capable of realizing small diameter product. The current industry standard diameter is ⅜ inches, although some manufacturers use 7 mm. Other manufacturers use 5 mm coils to produce heat exchangers having short lengths, for example no longer than 36 inches. However, when the hairpin tubing becomes too small, both the lacing process and the expansion process become exceedingly difficult, and commercially viable manufacturing of any but the smallest heat exchangers has previously not been possible. For example, heat exchangers six or more feet in length are readily manufactured using ⅜ inch copper tubing. However, when 5 mm copper tubing is used, before the present invention, it has not been commercially feasible to manufacture a heat exchanger longer than about 36 inches—the 5 mm copper tubing is too flimsy to readily lace and expand long heat exchangers, and the concomitant manufacturing time is too long to justify the expense of producing the 5 mm heat exchanger.
It is desirable, therefore, to provide a manufacturing process and system that allows tube and fin heat exchangers characterized by small diameter hairpin tubing, such as 5 mm copper tubing, to be quickly, easily, and cost-effectively manufactured.